Control of exhaust emissions from an internal combustion engine

ABSTRACT

Exhaust gas emissions from an internal combustion engine are controlled through the use of a heat exchanger, a sulfur oxide absorbent, and a catalytic converter. The exhaust gas is contacted with the sulfur oxide absorbent before it is passed to the catalytic converter, and the heat exchanger is used to extract heat from the exhaust gas before the exhaust gas is contacted with the sulfur oxide absorbent. The heat extracted from the exhaust gas by the heat exchanger is then used to heat the catalytic converter. In a preferred embodiment, the exhaust gas is also contacted with a hydrocarbon adsorbent which: (1) adsorbs organic compounds, such as hydrocarbons, from the exhaust gas at the low temperatures which are typical of an engine cold-start, and (2) desorbs them at the higher temperatures which are reached after sustained engine operation.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for the control ofexhaust gas emissions from an internal combustion engine. Moreparticularly, it relates to the use of a sulfur oxide absorbent incombination with a catalytic converter wherein indirect heat exchange isused to improve the combined performance of the absorbant and converter.

BACKGROUND OF THE INVENTION

The exhaust gas from a spark ignition or diesel engine which is operatedon a hydrocarbon fuel will typically contain small amounts of carbonmonoxide, organic compounds (such as hydrocarbons), nitrogen oxides andsulfur oxides which are undesirable emissions. The carbon monoxide andorganic compounds result from incomplete combustion of the fuel, thenitrogen oxides are primarily a result of the combination of nitrogenand oxygen from the air which is used to burn the fuel in the engine,and the sulfur oxides result primarily from sulfur which is present asan impurity in the fuel. Small amounts of sulfur oxides can also resultfrom sulfur compounds that may be present as components of the engine'slubricating oil.

Emission control systems for vehicles which are powered by an internalcombustion engine typically include a catalytic converter which is usedto catalyze the conversion of harmful emissions in the engine exhaust togases which are less objectionable. Catalysts have been developed foruse in catalytic converters which are highly effective in catalyzing:(1) the oxidation of organic compounds to water and carbon dioxide; (2)the oxidation of carbon monoxide to carbon dioxide; and (3) theconversion of nitrogen oxides to less objectionable products. However,the performance of such catalysts is often adversely affected by sulfurcompounds which can act as catalyst poisons.

Sulfur oxide emissions can be controlled by reducing the amount ofsulfur which is contained in the fuel. This approach will be used inCalifornia where the California Air Resources Board has establishedPhase 2 specifications for reformulated gasoline which become effectivein March of 1996. These Phase 2 specifications define a maximum sulfurcontent for gasoline of 40 ppm as a flat limit for producers whereas,previously, conventional gasoline in California could contain up to 150ppm of sulfur. Unfortunately, this approach is ultimately limited by thehigh cost of producing fuels which have a very low sulfur content.

It has been recognized for many years that the performance of a catalystwhich is used to promote the destruction of hydrocarbons, carbonmonoxide and nitrogen oxides in the exhaust gas from an engine can beimproved if the catalyst is protected from catalyst poisons that can bepresent in the exhaust gas. For example, U.S. Pat. Nos. 3,443,886 and3,429,656, both to Taylor et al. teach the use of a preconditioning zonecontaining a guard material which chemically reacts with and removesexhaust gas components, such as sulfur oxides, which would otherwisedeactivate the catalyst. It is disclosed that a suitable guard materialwill contain a combination of 45 to 90 parts by weight of calcium asCaO, 5 to 30 parts by weight of SiO₂ and 5 to 25 parts by weight ofsodium as Na₂ O. It is further disclosed in the Taylor et al.--656patent that the CaO--Na₂ O--SiO₂ guard materials can be made moreeffective when used in association with oxides of metals from the groupconsisting of Cu, Mn, V, Cr, Fe, Co, Ni and Mo.

British Patent Specification No. 1,444,444 discloses that when acatalyst is used to promote the afterburning of exhaust gases from anengine, lead compounds and sulfur in the fuel and also phosphorous andzinc in the engine lubricating oil have an adverse affect on thecatalyst performance. It is further disclosed that catalyst poisons inthe exhaust gases can be removed by passing the exhaust gases through anabsorptive material for such poisons prior to passage through thecatalyst. The absorptive material is preferably formed of porous aluminapellets which are capable of absorbing lead compounds in the exhaustgases. It is disclosed that sulphur or phosphorous can also be absorbedby adding a substance which is reactive with sulfur or phosphorous.

Published European Patent Application No. 0 582 917 A1 (Goto et al.) isdirected to an exhaust gas purification device for an engine wherein anNO_(x) absorbent is used to absorb NO_(x) emissions in the exhaust gas.It is disclosed that sulfur oxides in the exhaust gas have an adverseeffect on the ability of the NO_(x) absorbent to absorb NO_(x). It isfurther disclosed that a sulphur trapping absorbent can be placed in theexhaust gas upstream of the NO_(x) absorbent for the purpose ofpreventing sulfur oxides from flowing into the NO_(x) absorbent. TheSO_(x) absorbent contains at least one substance selected from alkalimetals, alkali-earth metals, rare-earth metals, and precious metals suchas platinum. Alumina can be used as a carrier for the SO_(x) absorbent.

Unfortunately, modern catalytic converters only operate after reachingtemperatures in excess of about 300° C. For this reason, a substantialportion of hydrocarbon emissions from an internal combustion engineusually occur during the first few minutes of cold-start engineoperation before the converter reaches its minimum effective operatingtemperature. This minimum effective operating temperature is frequentlyreferred to as the converter "light-off" temperature. Because the firstfew minutes of operation are an integral part of automotive emissionstests, and because over 60% of the measured hydrocarbons can be emittedduring the cold-start period of the test, a reduction of cold-starthydrocarbon emissions is of critical importance. Recent tightening ofemissions requirements to limit emissions of certain hydrocarboncompounds, such as benzene, has further underscored the need for reducedcold-start hydrocarbon emissions.

The use of a sulfur absorbent to remove sulfur oxides from an exhaustgas before they can reach and poison a catalytic converter is aneffective way to protect and maintain the activity and lifetime of thecatalytic converter which is used to catalyze the conversion of carbonmonoxide, organic compounds, and nitrogen oxides to less objectionableproducts. Although the use of such an absorbent solves one problem, italso creates an entirely new problem. A large quantity of absorbent mustbe used in view of the fact that it must have the ability to absorbsignificant quantities of sulfur oxides. By way of illustration, if agasoline containing 150 ppm of sulfur and having a density of 6.5 poundsper gallon (0.7789 g/cm³) is used as the fuel for an automobile that hasan overall fuel economy of 20 miles per gallon (8.50 km/l), a total of0.488 pounds (221 g) of sulfur will be discharged in the exhaust gas forevery 10,000 miles (16,100 km) of operation. Accordingly, a significantmass of absorbent will be required to capture this sulfur. Because ofits mass, the absorbent will absorb a significant amount of heat fromthe exhaust gases upon cold-start of the engine, and the presence ofthis heat sink will increase the amount of time required before thecatalytic converter reaches its "light-off" temperature. Accordingly,the use of a sulfur oxide absorbent to protect the catalytic converterin accordance with the teaching of the prior art will cause anundesirable increase in cold-start hydrocarbon emissions.

The problem of cold-start hydrocarbon emissions has been addressedthrough the use of adsorbents which have the ability to adsorbhydrocarbons from the exhaust gas at low temperatures and then releasethe adsorbed hydrocarbons at higher temperatures when the catalyticconverter has reached its "light-off" temperature. For example, U.S.Pat. Nos. 5,158,753 (Take et al.) and 5,303,547 (Mieville et al.)disclose processes wherein an adsorbent is used to adsorb organicsubstances from the exhaust gas in combination with a heat exchangerwhich transfers heat from the exhaust gas to the catalytic converter bymeans of a heat exchanger before the exhaust gas is passed through theadsorbent. At the low temperatures of a cold-start, the hydrocarbonadsorbent adsorbs hydrocarbons from the exhaust gas. As the hydrocarbonadsorbent is heated to high temperatures by continued exposure to theexhaust gas, the adsorbed hydrocarbons are desorbed and then passed tothe catalytic converter which has been heated by indirect heat exchange.

Published International Patent Application No. WO 94/11623 (Burk et al.)is also directed to the use of a hydrocarbon adsorbent to reducecold-start hydrocarbon emissions from an engine. More specifically, itdiscloses the treatment of an engine exhaust gas through the use of afirst and a second catalyst zone and a hydrocarbon adsorbent zonebetween them, wherein the first and second catalyst zones are in heattransfer relation to one another. Heat transfer from the first catalystzone to the second catalyst zone helps to bring the second catalysttherein more quickly to its effective operating temperature, and thehydrocarbon adsorbent in the adsorbent zone reduces the quantity ofhydrocarbons discharged to the atmosphere during engine cold-start byadsorbing hydrocarbons from the exhaust gas until the second catalystreaches a temperature at which it can more effectively convert thehydrocarbons to innocuous substances. This publication also teaches thatair can be added to the exhaust gas stream at a point upstream of thesecond catalyst zone.

SUMMARY OF THE INVENTION

A variety of emission control systems have been developed for vehicleswhich are powered by an internal combustion engine. Some of thesesystems are quite effective for the control of hydrocarbon and carbonmonoxide emissions through the use of catalytic converters. However,there is a continued need for an emission control system which is evenmore efficient in controlling exhaust gas emissions. In particular,there is a need for an emission control system which removes sulfuroxide emissions, protects the catalytic converter from deactivation bysulfur oxides, and minimizes hydrocarbon emissions during enginecold-start periods. We have found that such a system can be constructedthrough the use of: (1) a sulfur oxide absorbent to treat the exhaustgas before it is passed to the catalytic converter; and (2) a heatexchanger to transfer heat from the exhaust gas to the catalyticconverter before said exhaust gas contacts the sulfur oxide absorbent.

One embodiment of the invention is a method for reducing theconcentration of carbon monoxide, organic compounds and sulfur oxides inan exhaust gas from an internal combustion engine which comprises: (a)contacting the exhaust gas with a sulfur oxide absorbent in a firstcontacting zone and absorbing with the sulfur oxide absorbent at least aportion of the sulfur oxides in the exhaust gas wherein said sulfuroxide absorption is substantially irreversible at temperatures which areless than or equal to that of said exhaust gas; (b) contacting theeffluent gas from said first contacting zone with a catalyst in a secondcontacting zone and catalyzing the conversion of at least a portion ofthe carbon monoxide and organic compounds in the effluent gas from saidfirst contacting zone to innocuous products; and (c) transferring heatfrom the exhaust gas to said second contacting zone by indirect heatexchange.

Another embodiment of the invention is a method for reducing theconcentration of carbon monoxide, organic compounds and sulfur oxides inthe exhaust gas from an internal combustion engine which comprises: (a)contacting said exhaust gas with a first catalyst in a first contactingzone and catalyzing the conversion of at least a portion of the carbonmonoxide and organic compounds to innocuous products with said firstcatalyst; (b) contacting the effluent gas from said first contactingzone with a sulfur oxide absorbent in a second contacting zone andabsorbing with the sulfur oxide absorbent at least a portion of thesulfur oxides in said effluent gas from said first contacting zone; (c)contacting the effluent gas from said second contacting zone with asecond catalyst in a third contacting zone and catalyzing the conversionof at least a portion of residual carbon monoxide and organic compoundsin the effluent gas from said second contacting zone to innocuousproducts; and (d) heating said third contacting zone with heat that isremoved from said first contacting zone by indirect heat exchange.

Another embodiment of the invention is an apparatus for reducing theconcentration of carbon monoxide, organic compounds and sulfur oxides inan exhaust gas from an internal combustion engine which comprises: (a)absorber means which comprises a sulfur oxide absorbent for absorbing atleast a portion of the sulfur oxides in the exhaust gas in asubstantially irreversible manner at temperatures which are less than orequal to that of said exhaust gas; (b) catalytic converter means forreceiving effluent gas from the absorber means, wherein said catalyticconverter means comprises a catalyst which is effective for catalyzingthe conversion of hydrocarbons into innocuous products; and (c) heatexchange means for transferring heat from the exhaust gas to thecatalytic converter means by indirect heat exchange before said exhaustgas enters the absorber means.

A further embodiment of the invention is an apparatus for reducing theconcentration of carbon monoxide, organic compounds and sulfur oxides inthe exhaust gas from an internal combustion engine which comprises: (a)first catalytic converter means comprising a catalyst which is effectivefor catalyzing at least a portion of the carbon monoxide and organiccompounds to innocuous products; (b) absorber means for receivingeffluent gas from the first catalytic converter means, wherein saidabsorber means comprises a sulfur oxide absorbent for absorbing at leasta portion of the sulfur oxides in the exhaust gas in a substantiallyirreversible manner at temperatures which are less than or equal to thatof said exhaust gas; (c) second catalytic converter means for receivingeffluent gas from the absorber means, wherein said second catalyticconverter means comprises a catalyst which is effective for catalyzingthe conversion of at least a portion of residual carbon monoxide andorganic compounds in the effluent gas from the absorber means toinnocuous products; and (d) heat exchange means for transferring heatfrom the first catalytic converter means to the second catalyticconverter means by indirect heat exchange.

An object of the invention is to provide a method and apparatus for theimproved removal of sulfur oxide emissions from the exhaust gas of aninternal combustion engine.

An object of the invention is to provide an improved method andapparatus for the control of carbon monoxide, hydrocarbon, and sulfuroxide emissions in the exhaust gas from an internal combustion engine.

Another object of the invention is to provide a method and apparatus forprotecting a catalytic converter from sulfur compounds which does notenhance the release of hydrocarbon emissions during engine cold-startperiods.

A further object of the invention is to provide a method and apparatusfor the removal of sulfur oxide emissions from an engine exhaust gaswhich simultaneously reduces hydrocarbon emissions during enginecold-start periods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the drawings is a schematic representation of a firstembodiment of the invention.

FIG. 2 of the drawings is a perspective, partially cut away view of acrossflow monolith which can be used as a heat exchanger in the practiceof the invention.

FIG. 3 of the drawings is a schematic representation of a secondembodiment of the invention.

FIG. 4 of the drawings is a schematic representation of a thirdembodiment of the invention.

FIG. 5 of the drawings is a schematic representation of a fourthembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

While this invention is susceptible of embodiment in many forms,specific embodiments are schematically shown in FIGS. 1 and 3-5, withthe understanding that the present disclosure is not intended to limitthe invention to the embodiments which are illustrated.

With reference to FIG. 1, exhaust gas from engine 1 flows through heatexchanger 2 by means of line 3 where heat is transferred from the hotexhaust gas to heat exchanger 2. The cooled exhaust gas then flows fromheat exchanger 2 through line 4 to a sulfur oxide absorption zone 5. Theexhaust gas then passes through sulfur oxide absorption zone 5 wheresulfur oxides are removed from the exhaust gas through contact with asulfur oxide absorbent. Effluent gas from sulfur oxide absorption zone 5passes through line 6 to heat exchanger 2 where it is heated by the heatthat it previously gave up during its initial passage through heatexchanger 2 via line 3. The heated exhaust gas is then discharged fromheat exchanger 2 through line 7 and passed to catalytic converter 8. Atleast a portion of any carbon monoxide and organic compounds in theexhaust gas are catalytically converted to innocuous products incatalytic converter 8, and the resulting exhaust gas is discharged fromcatalytic converter 8 through line 9.

Engine 1 can be any internal combustion engine that produces an exhaustgas that contains sulfur oxides. For example, the invention isparticularly useful in treating the exhaust gas from a spark ignitioninternal combustion engine which is operated with a gasoline fuel whichcontains organic sulfur compounds as impurities. The invention can alsobe used with a diesel engine which is operated with a fuel that containssulfur compounds as impurities.

The invention can be used to process an exhaust gas that contains a widerange of sulfur oxide concentrations, for example, from less than about0.01 ppm up to about 1%. However, the invention is best employed withexhaust gases that contain less than about 20 ppm of sulfur oxides. Theinvention is suitable for use in controlling sulfur oxide emissions froma gasoline operated automotive engine wherein the gasoline contains lessthan about 300 ppm of sulfur in the form of organic sulfur compounds.Preferably, the gasoline contains less than about 150 ppm and morepreferably less than about 50 ppm of sulfur. These sulfur levels aresignificant only because they are a measure of the amount of sulfur thatmust be captured by sulfur oxide absorption zone 5, and of the quantityof sulfur oxide absorbent that must be used for this purpose in zone 5.For example, if a gasoline containing 50 ppm of sulfur and having adensity of 6.5 pounds per gallon (0.7789 g/cm³) is used as the fuel foran automobile that has an overall fuel economy of 20 miles per gallon(8.50 km/l), a total of 0.325 pounds (147 g) of sulfur will bedischarged in the exhaust gas for every 20,000 miles (32,200 km) ofoperation. However, if the gasoline contains 150 ppm of sulfur, threetimes as much sulfur will be discharged in the exhaust gas. Accordingly,the invention provides a highly satisfactory and inexpensive method forthe substantially complete elimination of sulfur oxide emissions fromautomotive engines that are operated on a low sulfur gasoline such asCalifornia Phase 2 reformulated gasoline which cannot have a sulfurcontent higher than 40 ppm.

If the exhaust gas from an engine were passed directly to a sulfur oxideabsorbent and then to a catalytic converter, there would be a seriouscold-start emission problem. Because of the thermal mass of the coldabsorbent in the sulfur oxide absorption zone, the temperature of theexhaust gas will be drastically reduced by direct heat transfer to theabsorbent during passage through this zone. Accordingly, during asignificant period of time after a cold-start, effluent gases from thesulfur oxide absorption zone will be too cold to bring the catalyticconverter to its light-off temperature. More specifically, the sulfuroxide adsorption zone will have to be first heated to the light-offtemperature of the catalytic converter before its effluent gases are hotenough to bring the catalytic converter to this temperature. Theconsequence of this is a significant delay before the catalyticconverter reaches its light-off temperature and an associated failure bythe catalytic converter to destroy organic compounds (such ashydrocarbons) in the exhaust gas during the delay period.

In the practice of the present invention, heat exchanger 2 is used toremove heat from the engine exhaust gas before the gas is passed tosulfur oxide absorption zone 5 for removal of sulfur oxides. This heatis then put back into the exhaust gas after it is discharged from sulfuroxide absorption zone 5. As a consequence, catalytic converter 8 isbrought to its light-off temperature more rapidly during cold-startperiods. The effect of heat exchanger 2 is to eliminate a significantportion of the undesirable increase in cold-start hydrocarbon emissionsthat would otherwise be caused by inserting the thermal mass of thesulfur oxide absorption zone in front of the catalytic converter.

Heat exchanger 2 can be of any known design and can be constructed ofany material which has suitable mechanical and thermal conductivityproperties and is tolerant to the thermal and chemical environmentcreated by a hot exhaust gas. Suitable materials include, but are notlimited to, stainless steel and ceramics. A suitable heat exchanger 2can, for example, comprise a first plurality of heat exchange passageswhich is in heat exchange relationship with a second plurality of heatexchange passages, wherein one set of heat exchange passages is used forthe hot gases and the other is used for the cooler gases.

A highly suitable structure for heat exchanger 2 is illustrated in FIG.2. The structure of FIG. 2 is a crossflow monolith wherein horizontalpassages 11 provide a first plurality of heat exchange passages andvertical passages 12 provide a second plurality of heat exchangepassages, wherein the two sets of passages are in heat exchangerelationship with each other. The passages of these two sets of heatexchange passages are disposed at right angles to each other, andadjacent passages 11 and 12 are separated by a thin wall 13. Thestructure of FIG. 2 can be formed by cementing together alternating,perpendicular rows of extruded ceramic ducts. Alternatively, similarlyshaped metallic or ceramic-coated metallic structures may be producedand joined together by cementing or welding as appropriate.

The sulfur oxide absorbent used in sulfur oxide absorption zone 5 can besubstantially any solid material that is capable of absorbing sulfuroxides at exhaust gas temperatures to yield a product which is thermallystable at exhaust gas temperatures. That is to say, the absorption ofsulfur oxides from the exhaust gas by the sulfur oxide absorbent must bea process which is substantially irreversible thermally at thetemperatures of its use. Preferably, the sulfur oxide absorbent willhave a higher thermal decomposition temperature than any temperature itis expected to experience. The temperature of the exhaust gases from aconventional gasoline powered, spark ignition, internal combustionautomotive engine when they reach absorption zone 5 can be at or nearambient temperature during cold-start periods and can be as high asabout 800° C. during periods of sustained engine operation.

A highly suitable sulfur oxide absorbent for use in the practice of thisinvention is comprised of at least one metal oxide selected from theoxides of calcium, magnesium, strontium, barium and zinc. This sulfuroxide absorbent can also additionally comprise at least one oxideselected from the group consisting of the oxides of aluminum, silicon,copper, manganese, sodium, vanadium, iron, chromium, and the rare earthmetals such as cerium, praseodymium, samarium and dysprosium. The sulfuroxide absorbent can also advantageously contain a small amount of ametal selected from the group consisting of ruthenium, rhodium,palladium, osmium, iridium, platinum and rhenium. In a preferredembodiment, the sulfur oxide absorbent will comprise a combination of:(1) at least one metal oxide selected from the oxides of calcium,magnesium, strontium, barium and zinc; and (2) at least one oxideselected from the oxides of aluminum and silicon; and (3) at least onemetal selected from the group consisting of vanadium, iron, chromium,the platinum group metals (ruthenium, rhodium, palladium, osmium,iridium and platinum), rhenium and the rare earth metals. Although theinvention is not to be so limited, it is believed that the presence inthe sulfur oxide absorbent of a metal such as vanadium, iron, chromium,a platinum group metal, rhenium or a rare earth metal serves to enhancethe efficiency of the absorbent by catalyzing the oxidation of sulfurdioxide in the exhaust gas to sulfur trioxide which is more easilycaptured by the absorbent.

By way of example, compositions consisting of a combination of 45 to 90parts by weight of calcium as CaO, 5 to 30 parts by weight of SiO₂, and5 to 25 parts by weight of sodium as Na₂ O are suitable sulfur oxideabsorbents for use in the practice of this invention. The preparation ofsuch materials is disclosed in U.S. Pat. No. 3,443,886 (Taylor et al.).In addition, such materials can be promoted by combination with a metaloxide selected from the group consisting of the oxides of Cu, Mn, V, Cr,Fe, Co, Ni and Mo. These promoted materials are also suitable for use inthe practice of this invention, and their preparation is disclosed inU.S. Pat. No. 3,429,656 (Taylor et al.).

Unexpectedly, we have found that many of the materials that have beendeveloped to suppress the emission of sulfur oxides from catalystregeneration in the fluidized catalytic cracking ("FCC") process arehighly suitable for use as a sulfur oxide absorbent in the practice ofthis invention. The fluidized catalytic cracking process is one of theprincipal industrial processes currently used for the manufacture ofgasoline. In this process, a high boiling point hydrocarbon feedstock isconverted to a lower boiling point product by contact with a crackingcatalyst at elevated temperatures. The catalyst becomes deactivated inthe course of the process as a result of coke deposits that form on thecatalyst. Accordingly, the catalyst must be periodically regenerated bycombustion of these coke deposits. This regeneration process results inthe formation of an effluent gas which contains sulfur oxides, and thequantity of sulfur oxides produced is a function of the sulfur contentof the hydrocarbon feedstock. Certain catalyst additives have beendeveloped which can be added to the cracking catalyst which are highlyeffective in suppressing the discharge of sulfur oxides from catalystregeneration. It is believed that these additives function by absorbingsulfur oxides in the oxidizing environment of the regeneration stage ofthe process and releasing the absorbed sulfur oxides, perhaps ashydrogen sulfide, in the reducing environment of the reaction zone wherethe hot cracking catalyst is contacted with hydrocarbon feedstock. Ineffect, these catalyst additives for the FCC process are used totransport sulfur from the regeneration zone to the reaction zone.Accordingly, it is surprising that these FCC catalyst additives arehighly suitable for use in the practice of this invention.

Highly satisfactory sulfur oxide absorbents for use in this inventioncomprise an inorganic oxide composition in association with at least onefree or combined metal selected from the group consisting of vanadium,iron, lanthanum, cerium, praseodymium, samarium and dysprosium, whereinthe inorganic oxide composition comprises at least one member selectedfrom the group consisting of mixtures of magnesium oxide with aluminumoxide, MgAl₂ O₃ xAl₂ O₃, and MgAl₂ O₄.yMgO wherein x is from 0 to 10 andy is from 0 to 10. The ratio by weight of the inorganic oxidecomposition to the metal or metals in such a material is preferably inthe range from about 1.0 to about 1,000. Materials of this type havebeen utilized as catalyst additives for the FCC process, and thepreparation of such materials is disclosed in U.S. Pat. Nos. 4,469,589(Yoo et al.), 4,472,267 (Yoo et al.), and 4,497,902 (Bertolacini etal.). Such a material which consists of cerium and vanadium incombination with MgAl₂ O₄.MgO has been found to have the ability toincrease its weight by 47% through absorption of sulfur dioxide at atemperature of about 735° C. from a mixture of gases consisting of 5000ppm of sulfur dioxide and 2% oxygen in helium.

Another highly satisfactory sulfur oxide absorbent for use in thisinvention comprises a material of the formula:

    M.sub.2m.sup.2 +Al.sub.2-p M.sub.p.sup.3+ T.sub.r O.sub.7+r(s)

where m is about 1.1 to about 3.5; p is about 0.01 to about 0.4; M²⁺ isa divalent metal selected from the group consisting of magnesium,calcium, zinc, barium and strontium; M³⁺ is a trivalent metal selectedfrom the group consisting of cerium, lanthanum, iron, chromium, vanadiumand cobalt; T is vanadium, tungsten or molybdenum; r is about 0.01 toabout 0.2; and s is 2.5 when T is vanadium or 3 when T is tungsten ormolybdenum. Materials of this type have also been suggested for use ascatalyst additives for the FCC process, and their preparation isdisclosed in U.S. Pat. No. 5,426,083 (Bhattacharyya et al.) Such amaterial, which is described in the Bhattacharyya et al. patent, is ofthe formula:

    Mg.sub.4 Al.sub.1.796 Ce.sub.0.204 V.sub.0.074 O.sub.7.185

and has the ability to increase its weight by 84.6% through absorptionof sulfur dioxide at a temperature of about 735° C. from a mixture ofgases consisting of 5000 ppm of sulfur dioxide and 2% oxygen in helium.

The suitability of a variety of metal oxides and carbonates (includingthose of sodium, potassium, magnesium, calcium, barium, aluminum,manganese, iron, copper and zinc) as absorbents for sulfur trioxideemissions in automotive exhaust gases has been extensively discussed byWilliam R. Leppard in report No. EPA-460/3-75-002-a prepared for theU.S. Environmental Protection Agency under Contract No. 68-03-0497 anddated November 1974. The teaching of this reference is herebyincorporated by reference.

The sulfur oxide absorbent can be used in any desired form. For example,it can be used in the form of particles, pellets, beads or monoliths. Inaddition, the sulfur oxide absorbent can be either placed on a supportor incorporated into another material which can serve either as a binderor a support. Conventional ceramic supports can be used for thispurpose. Silica and alumina can be used either as a support or as abinder for the sulfur oxide absorbent. For example, silica and/oralumina can be advantageously used either as a support or binder formagnesium oxide or calcium oxide. If desired, the absorbent can be usedin the form of pellets or beads disposed in a flow-through canister toprovide a bed of absorbent through which the exhaust gas flows.Desirably, the sulfur oxide absorbent will be used in a physical formwhich will permit a rapid and effective contacting with the exhaust gaswithout creating substantial amounts of back-pressure which canadversely affect engine performance. Preferably, the sulfur oxideabsorbent will comprise a particulate solid wherein the largestdimension of the particles has an average value which is in excess ofabout 1 millimeter. For example, pellets or rods of about 0.10 to 0.60cm diameter and 0.25 to 2.54 cm length can be used in the practice ofthe invention.

The amount of sulfur oxide absorbent used in sulfur oxide absorptionzone 5 is preferably effective to absorb an amount of sulfur oxides,calculated on the basis of sulfur content, which is equal to at leastabout 100 grams of sulfur. More preferably, the amount of sulfur oxideabsorbent used in zone 5 will be effective to absorb an amount of sulfuroxides, calculated on the basis of sulfur content, which is equal to atleast about 200 grams of sulfur. In addition, the sulfur oxide absorbentshould be effective to absorb an amount of sulfur oxides, calculated onthe basis of sulfur content, which is desirably equal to at least about10% and preferably equal to at least about 20% of the absorbent'sweight. Desirably, the sulfur oxide absorbent is effective to absorb atleast about 50% of the sulfur oxides which are passed into sulfur oxideabsorption zone 5. Preferably, at least about 80% of the sulfur oxidespassed into sulfur oxide absorption zone 5 are absorbed by the sulfuroxide absorbent.

The sulfur oxide absorption zone 5 can be constructed of any materialwhich has suitable mechanical properties and is tolerant to the thermaland chemical environment created by a hot exhaust gas. Suitablematerials include, but are not limited to, stainless steel and ceramics.If desired, the sulfur oxide absorption zone 5 can be constructed forconvenient replacement.

If desired, air can be added to the exhaust gas before it enters sulfuroxide absorption zone 5. It will be appreciated, of course, that theadded air will serve to cool the exhaust gas. Accordingly, it ispreferred to extract heat from the exhaust gas with heat exchanger 2before adding any such supplemental air to the exhaust gas. Desirably,the amount of air used will be at least enough, on a stoichometricbasis, to oxidize any oxidizable carbon containing materials (such ascarbon monoxide and organic compounds) to carbon dioxide and water.Although the invention is not to be so limited, it is believed that thepresence of excess oxygen serves to enhance the ability of the sulfuroxide absorbent in zone 5 to absorb sulfur oxides because this excessoxygen promotes the conversion of sulfur dioxide to sulfur trioxidewhich is more easily absorbed. For the same reason, it is believed thatthe presence in the sulfur oxide absorbent of a metal such as vanadium,iron, chromium, a platinum group metal, rhenium or a rare earth metalserves to enhance the efficiency of the absorbent by catalyzing theoxidation of sulfur dioxide in the exhaust gas to sulfur trioxide whichis more easily captured by the absorbent.

The addition of air to the exhaust gas before it enters sulfur oxideabsorption zone 5 can be advantageous when the engine is operated with arich air/fuel mixture to produce an exhaust gas which is reducing incharacter. The addition of air to such a reducing exhaust gas can beused to minimize or prevent any possible release of a sulfur containinggas, such as hydrogen sulfide, which might occur by reduction of sulfuroxides which have been absorbed by the sulfur oxide absorbent inabsorption zone 5. Such supplemental air can, of course, also react withthe reducing components of the exhaust gas and thereby prevent theirdischarge as undesirable emissions. Any air pump or blower ofconventional design can be used to provide the supplemental air.

In a highly preferred embodiment of the invention, a hydrocarbonadsorbent is used in combination with the sulfur oxide absorbent. Insuch an embodiment, sulfur oxide absorption zone 5 can additionallycomprise the hydrocarbon adsorbent. Alternatively, the hydrocarbonadsorbent can be placed in a supplemental contacting zone which islocated either upstream or downstream of sulfur oxide absorption zone 5,wherein "upstream" and "downstream" refer to the flow of the exhaustgas, and upstream indicates a location in the exhaust gas before the gaspasses through zone 5, while downstream indicates a location in theexhaust gas after its passage through zone 5. However, it will beappreciated that the supplemental hydrocarbon adsorption zone should belocated at a position downstream from the "hot side" of heat exchanger 2but upstream from "cold side" of heat exchanger 2, wherein "hot side"refers to the side of the heat exchanger that receives hot exhaust gasbefore it is passed to either sulfur oxide absorption zone 5 or thesupplemental hydrocarbon adsorption zone. Similarly, "cold side" refersto the side of the heat exchanger that receives the exhaust gas afterpassage through sulfur oxide absorption zone 5 and the supplementalhydrocarbon adsorption zone. FIG. 3 illustrates that embodiment of theinvention wherein a supplemental hydrocarbon adsorption zone 16 islocated downstream of sulfur oxide absorption zone 5.

With reference to FIG. 3, exhaust gas from engine 1 flows through line 3to heat exchanger 2 where heat is transferred from the hot exhaust gasto heat exchanger 2. The cooled exhaust gas then flows from heatexchanger 2 through line 4 to sulfur oxide absorption zone 5. Theexhaust gas then passes through sulfur oxide absorption zone wheresulfur oxides are removed from the exhaust gas through contact with asulfur oxide absorbent. Effluent gas from sulfur oxide absorption zone 5passes through line 15 to hydrocarbon adsorption zone 16 where theexhaust gas is contacted with a hydrocarbon adsorbent. Effluent gas fromhydrocarbon adsorption zone 16 is passed through line 17 to heatexchanger 2 where it is heated by the heat that it previously gave upduring its initial passage through heat exchanger 2 via line 3. Theheated exhaust gas is then discharged from heat exchanger 2 through line7 and is passed to catalytic converter 8 where it is contacted with acatalyst which can catalyze the conversion of carbon monoxide andorganic compounds in the exhaust gas to innocuous products. Finally, theresulting exhaust gas is discharged from catalytic converter 8 throughline 9.

As previously noted, the effect of heat exchanger 2 is to eliminate asignificant portion of the undesirable increase in cold-starthydrocarbon emissions that would otherwise be caused by inserting thethermal mass of the sulfur oxide absorption zone in front of thecatalytic converter. However, by itself, heat exchanger 2 cannot totallyeliminate the discharge of hydrocarbon emissions during enginecold-start periods because catalytic converter 8 cannot be broughtimmediately to its light-off temperature. Accordingly, the use of ahydrocarbon adsorbent in the practice of this invention is a highlypreferred embodiment because the hydrocarbon adsorbent serves to: (1)adsorb at least a portion of the organic compounds in the exhaust gasunder the low temperature conditions which are characteristic of enginecold-start, and (2) desorb the adsorbed organic compounds under thehigher temperature conditions which result after the engine has operatedbriefly and the catalytic converter has reached its light-offtemperature. The combined use of a hydrocarbon adsorbent with a sulfuroxide absorbent in the practice of this invention makes possible thehighly effective control of both sulfur oxide emissions and cold-starthydrocarbon emissions. In the embodiment illustrated in FIG. 3, sulfuroxide absorption zone 5 is located upstream from from supplementalhydrocarbon adsorption zone 16. In this arrangement, the thermal mass ofsulfur oxide absorption zone 5 enhances the ability of the hydrocarbonadsorbent in zone 16 to control cold-start hydrocarbon emissions.Because of this thermal mass, hydrocarbon adsorption zone 16 does notheat up as rapidly as it would in the absence of sulfur oxide absorptionzone 5. Accordingly, the hydrocarbon adsorbent can adsorb hydrocarbonsfor a longer period of time and catalytic converter 8 has this longerperiod of time to reach its light-off temperature.

The hydrocarbon adsorbent can be comprised of any of the known materialswhich are suitable for adsorbing organic compounds, such ashydrocarbons, from an exhaust gas at the low temperatures which aretypical of an engine cold-start and desorbing them at the highertemperatures which are reached after sustained engine operation. Suchmaterials include, but are not limited to, activated alumina, porousglass, silica gel, activated carbon, and both natural and syntheticmolecular sieves. Suitable molecular sieves include faujasite,mordenite, chabazite, silicalite, Beta zeolite, zeolite X, zeolite Y,and ZSM-5 zeolite. For example, Union Carbide ultrastable Y sieves suchas LZY-72 and LZY-82 are satisfactory hydrocarbon adsorbents. Whileactivated carbon is an excellent adsorbent, its use in this invention isnot preferred since it can be damaged by sustained exposure to a hightemperature exhaust gas. For this reason, activated carbon should not beused except where the exhaust gas constituents will not oxidize thecarbon significantly and where the hydrocarbon adsorbent temperature issufficiently low to ensure continued operability of a carbon adsorbent.

The hydrocarbon adsorbent can be used in any desired form. For example,it can be used in the form of particles, pellets, beads or monoliths. Inaddition, the hydrocarbon adsorbent can be either placed on a support orincorporated into another material which can serve either as a binder ora support. Conventional ceramic or metal supports can be used for thispurpose. Silica and alumina can be used either as a support or as abinder for the hydrocarbon adsorbent. For example, silica and/or aluminacan be advantageously used either as a support or binder for a molecularsieve such as a Beta zeolite. If desired, the adsorbent can be used inthe form of pellets or beads in a flow-through canister to provide anadsorbent bed through which the exhaust gas flows. Desirably, thehydrocarbon adsorbent will be used in a physical form which will permita rapid and effective contacting with the exhaust gas without creatingsubstantial amounts of back-pressure which can adversely affect engineperformance. Preferably, the hydrocarbon adsorbent will comprise aparticulate solid wherein the largest dimension of the particles has anaverage value which is in excess of about 1 millimeter. For example,pellets or rods of about 0.10 to 0.60 cm diameter and 0.25 to 2.54 cmlength can be used in the practice of the invention. In a highlypreferred embodiment, the hydrocarbon adsorbent is deposited on a solidmonolithic carrier. For example, the hydrocarbon adsorbent can beapplied as a thin film or coating on an inert carrier material whichprovides structural support for the adsorbent. The carrier material canbe any refractory material which is stable under the conditions createdby exposure to a hot exhaust gas. Such materials include conventionalceramics and metals. Conventional hydrocarbon adsorbents and their useto reversibly adsorb hydrocarbons from an exhaust gas are disclosed inU.S. Pat. No. 5,303,547 (Mieville et al.) and in Published InternationalPatent Application No. WO 94/11623 (Burk et al.).

With reference to FIG. 1, sulfur oxide absorption zone 5 can compriseboth a sulfur oxide absorbent and a hydrocarbon adsorbent. These twomaterials can be combined in any desired manner. For example, thehydrocarbon adsorbent can be a separate particulate solid which isphysically mixed with particles of the sulfur oxide absorbent. It willbe appreciated, of course, that these particles can be either pellets orbeads of an appropriate size. Alternatively, the hydrocarbon adsorbentand the sulfur oxide absorbent can be combined so that both materialsare contained in the same particles, pellets or beads. For example, asulfur oxide absorbent such as magnesium or calcium oxide can becombined with a hydrocarbon adsorbent such as an ultrastable Y zeoliteor a Beta zeolite using silica and alumina as a binder or matrix. Such acomposite can be used in the form of pellets or beads within aflow-through canister to provide a bed of the composite material throughwhich the exhaust gas flows. Desirably, the composite material will beused in a physical form which will permit a rapid and effectivecontacting with the exhaust gas without creating substantial amounts ofback-pressure which can adversely affect engine performance.

With reference to FIGS. 1 and 3, catalytic converter 8 can be of anyconventional type which is known to the art and will contain a catalystwhich is effective to catalyze the oxidation of at least a portion ofthe carbon monoxide and organic compounds, such as hydrocarbons, in theexhaust gas to innocuous products. Such catalysts generally comprise oneor more of the platinum group metals which consist of ruthenium,rhodium, palladium, osmium, iridium and platinum. Occasionally, theplatinum group metal or metals are combined with one or more additionalmetals. Such additional metals include, but are not limited to,chromium, copper, vanadium, cobalt, nickel and iron. Suitable catalystsinclude the type which are commonly referred to as "three-wayconversion" or "TWC" catalysts. The TWC catalysts can comprise, forexample, combinations of: (1) platinum and rhodium; or (2) palladium andrhodium; or (3) platinum, palladium and rhodium. The platinum groupmetals of the TWC catalyst can be deposited on any suitable refractorysubstrate. Such substrates include, but are not limited to, alumina,ceria and zirconia. The support can be of any desired form, for example,particles, pellets, beads or monoliths. The catalytic metals can becombined with a support either during or after preparation of thesupport. One method consists of impregnating a suitable support with anaqueous or organic solution or dispersion of a suitable compound orcompounds of the catalytic metal or metals. The impregnation can becarried out in any manner which will not destroy the structure of thesupport. After drying, the composite can be calcined, if desired.Alternatively, a suitable compound or compounds of the catalytic metalscan be combined with a support precursor prior to a physical formingstep such as extrusion. If desired, the catalytic metals can bedeposited on or in a monolithic support by wash-coating apreviously-prepared support or by mixing the catalytic metals into theceramic batch material prior to extrusion.

A highly preferred embodiment of the invention is schematicallyillustrated in FIG. 4. In this embodiment, a catalytic converter 20 isused which comprises a heat exchange structure. In this embodiment,exhaust gas from engine 1 flows through line 3 to catalytic converter20. The exhaust gas then flows through a first plurality of heatexchange passages 21 in catalytic converter 20. Exhaust gas is thenconveyed through line 18 to sulfur oxide absorption zone 5 and passedthrough this zone 5 where sulfur oxides are removed from the exhaust gasthrough contact with a sulfur oxide absorbent. Effluent gas from sulfuroxide absorption zone 5 passes through line 19 to catalytic converter 20where the gas flows through a second plurality of heat exchange passages22 which are in heat exchange relationship with the first plurality ofheat exchange passages 21. Finally, the resulting exhaust gas isdischarged from catalytic converter 20 through line 23. In thisembodiment, the catalyst or catalysts of the catalytic converter areplaced within either: (1) the second plurality of heat exchange passages22, or (2) both the first and second pluralities of heat exchangepassages 21 and 22. It will be appreciated, of course, that when acatalyst is used in both the first and second pluralities of heatexchange passages 21 and 22, one type of catalyst can be used inpassages 21 and a different type can be used in passages 22. Thecatalytic converter 20 of this embodiment is conveniently constructedfrom crossflow monolith such as that illustrated in FIG. 2. If desired,supplemental air can be added through line 24 and mixed with the exhaustgas before it is passed into sulfur oxide absorption zone 5.

With reference to the embodiment of the invention illustrated by FIG. 4wherein the catalyst of catalytic converter 20 is contained only in theplurality of second heat exchange passages 22, heat is efficientlyextracted from the hot engine exhaust gas and used to heat thecatalytically-active passages 22 through indirect heat exchange. Duringengine cold-start periods, the flow of exhaust gas through passages 21heats catalytically-active passages 22 and simultaneously lowers theexhaust gas temperature before the gas reaches sulfur oxide absorptionzone 5. In this manner, the catalyst is rapidly heated to its light-offtemperature, and cold-start hydrocarbon emissions are minimized.Simultaneously, sulfur oxides are removed in sulfur oxide absorptionzone 5, thereby preventing their discharge into the atmosphere aspollutants and also protecting the catalyst in heat exchange passages 22from the deactivating effect of these materials. As disclosed above, itis preferred to use a hydrocarbon adsorbent in combination with thesulfur oxide absorbent of zone 5, wherein the sulfur oxide absorptionzone 5 can additionally comprise the hydrocarbon adsorbent or,alternatively, the hydrocarbon adsorbent can be placed in a supplementalcontacting zone which is located either upstream or downstream of zone5.

With reference to the embodiment of the invention illustrated by FIG. 4,wherein the catalyst of catalytic converter 20 is contained in bothfirst and second pluralities of heat exchange passages 21 and 22, thecatalyst contained in passages 21 begins to function almost immediatelyupon contact with the hot engine gases to convert at least a portion ofthe carbon monoxide and organic compounds, such as hydrocarbons, toinnocuous products. In addition, the catalyst in passages 21 alsoimproves the ability of the sulfur oxide absorbent in zone 5 to capturesulfur oxides. Although the invention is not to be so limited, it isbelieved that the catalyst in passages 21 catalyzes the conversion ofsulfur dioxide to sulfur trioxide which is more easily captured by thesulfur oxide absorbent. However, during engine cold-start periods, someof these undesirable emissions will not be converted by the catalyst inpassages 21 because the catalyst has not reached its light-offtemperature. In addition, the catalyst in passages 21 will typicallyhave a somewhat reduced catalytic activity because of its exposure tosulfur compounds in the exhaust gas. Any organic compounds, such ashydrocarbons, and carbon monoxide that are not converted to innocuousproducts in passages 21 are subsequently presented to the catalyst inpassages 22 after it has been heated by indirect heat exchange and afterthe removal of sulfur oxides from the exhaust gas in sulfur oxideabsorption zone 5. The catalyst in passages 22 will be rapidly heated toits light-off temperature by indirect heat exchange, and it will also beprotected from the deactivating effect of sulfur oxides by sulfur oxideabsorption zone 5. As disclosed above, it is preferred to use ahydrocarbon adsorbent in combination with the sulfur oxide absorbent ofzone 5. For example, the sulfur oxide absorption zone 5 can additionallycomprise the hydrocarbon adsorbent or, alternatively, the hydrocarbonadsorbent can be placed in a supplemental contacting zone which islocated either upstream or downstream of zone 5.

The use of a hydrocarbon adsorbent in a hydrocarbon adsorption zone 16upstream of sulfur oxide absorption zone 5 is illustrated in FIG. 5. Inthis embodiment, exhaust gas from engine 1 flows through line 3 tocatalytic converter 20. The exhaust gas then flows through a firstplurality of heat exchange passages 21 in catalytic converter 20.Exhaust gas is then conveyed through line 27 to hydrocarbon adsorptionzone 16 where it is contacted with a hydrocarbon adsorbent. Effluent gasfrom hydrocarbon adsorption zone 16 flows through line 28 to sulfuroxide absorption zone 5 where it is contacted with a sulfur oxideabsorbent. Effluent gas from sulfur oxide absorption zone 5 passesthrough line 29 to catalytic converter 20 where the gas flows through asecond plurality of heat exchange passages 22 which are in heat exchangerelationship with the first plurality of heat exchange passages 21.Finally, the resulting exhaust gas is discharged from catalyticconverter 20 through line 23. If desired, supplemental air can be addedthrough line 30 to the exhaust gas before it is passed into sulfur oxideabsorption zone 5.

U.S. Pat. No. 5,303,547 (Mieville et al.) and Published InternationalPatent Application No. WO 94/11623 (Burk et al.) disclose catalyticconverters which comprise a heat exchanger. Catalytic convertersdisclosed in these references are suitable for use in the practice ofthe embodiments of this invention which are illustrated in FIGS. 4 and5, and these references are incorporated herein by reference.

The use of a first catalyst to contact the exhaust gas before it ispassed through the sulfur oxide absorbent and a second catalyst tocontact the effluent gas from the sulfur oxide absorbent permits the useof alternative gas treatment regimes in which the different componentsof a multifunction catalyst can be preferentially distributed throughoutdifferent regions of the exhaust gas treatment system. For example, inan embodiment of the type shown in FIGS. 4 and 5 wherein a catalyst isemployed in both first and second pluralities of heat exchange passages21 and 22, a nitrogen oxide reducing catalyst such as those that containrhodium, ruthenium or similar metals can be used in the plurality offirst heat exchange passages 21 and a platinum or standard three-wayconversion catalyst can be used in the plurality of second heat exchangepassages 22. In this embodiment, the engine can be operated with a richair/fuel mixture which will provide a reducing environment in the firstpassages 21 which will enhance the conversion of nitrogen oxides tonitrogen. Supplemental air can then be added to the exhaust gas beforeit is passed through the sulfur oxide absorbent or the second passages22. The oxygen provided by this supplemental air serves to enhance theabsorption of sulfur oxides by the sulfur oxide absorbent and alsoserves to ensure the effective conversion of carbon monoxide and organiccompounds, such as hydrocarbons, to innocuous products upon contact withthe catalyst in the second passages 22. If it is desired to use ahydrocarbon adsorbent which is damaged or rendered ineffective bycontinued exposure to oxygen, such as activated carbon, this hydrocarbonadsorbent can be placed in a hydrocarbon adsorption zone 16 which isupstream from the sulfur oxide absorption zone 5 as shown in FIG. 5. Thesupplemental air can then be added through line 30 to the effluent fromhydrocarbon adsorption zone 16 before the effluent is passed into sulfuroxide absorption zone 5 as illustrated in FIG. 5.

We claim:
 1. A method for reducing the concentration of carbon monoxide,organic compounds and sulfur oxides in an exhaust gas from an internalcombustion engine which comprises:(a) contacting the exhaust gas with asulfur oxide absorbent in a first contacting zone and absorbing with thesulfur oxide absorbent at least a portion of the sulfur oxides in theexhaust gas wherein said sulfur oxide absorption is substantiallyirreversible at temperatures which are less than or equal to that ofsaid exhaust gas; (b) contacting the effluent gas from said firstcontacting zone with a catalyst in a second contacting zone andcatalyzing the conversion of at least a portion of the carbon monoxideand organic compounds in the effluent gas from said first contactingzone to innocuous products; and (c) transferring heat from the exhaustgas to said second contacting zone by indirect heat exchange.
 2. Themethod of claim 1 wherein the sulfur oxide absorbent comprises at leastone metal oxide selected from the group consisting of the oxides ofcalcium, magnesium, strontium, barium and zinc.
 3. The method of claim 2wherein said sulfur oxide absorbent additionally comprises at least oneoxide selected from the group consisting of the oxides of aluminum,silicon, copper, manganese, sodium, vanadium, iron, chromium and therare earth metals.
 4. The method of claim 2 wherein the sulfur oxideabsorbent additionally comprises at least one metal selected from thegroup consisting of ruthenium, rhodium, palladium, osmium, iridium,platinum and rhenium.
 5. The method of claim 1 wherein the sulfur oxideabsorbent comprises an inorganic oxide composition in association withat least one additional material selected from the group consisting ofvanadium, iron, lanthanum, cerium, praseodymium, samarium anddysprosium, and said inorganic oxide composition comprises at least onemember selected from the group consisting of mixtures of magnesium oxidewith aluminum oxide, MgAl₂ O₄.xAl₂ O₃, and MgAl₂ O₄.yMgO wherein x isfrom 0 to 10 and y is from 0 to
 10. 6. The method of claim 5 wherein theratio by weight of inorganic oxide composition to said other materialsis from about 1.0 to about 1,000.
 7. The method of claim 1 whichcomprises flowing the exhaust gas through a first plurality of heatexchange passages which is in heat exchange relationship with a secondplurality of heat exchange passages which define the second contactingzone and contain the catalyst.
 8. The method of claim 1 wherein saidfirst contacting zone additionally contains a hydrocarbon adsorbent andthe method further comprises:(a) adsorbing with the hydrocarbonadsorbent at least a portion of the organic compounds in the exhaust gasunder the low temperature conditions in the first contacting zone whichare characteristic of engine cold-start; and (b) desorbing adsorbedorganic compounds from the hydrocarbon adsorbent under highertemperature conditions in the first contacting zone after the engine hasoperated for a period of time.
 9. The method of claim 8 wherein saidhydrocarbon adsorbent comprises a molecular sieve component.
 10. Themethod of claim 8 wherein the hydrocarbon adsorbent is a separateparticulate solid which is physically mixed with particles of the sulfuroxide absorbent.
 11. The method of claim 8 wherein the hydrocarbonadsorbent and the sulfur oxide absorbent comprise a particulate solidand are contained in the same particles.
 12. The method of claim 1wherein the amount of sulfur oxide absorbent in said first contactingzone is effective to absorb an amount of sulfur oxides, calculated onthe basis of sulfur content, which is equal to at least about 100 gramsof sulfur.
 13. The method of claim 1 wherein said sulfur oxide absorbentis effective to absorb at least about 50% of the sulfur oxides in saidexhaust gas.
 14. The method of claim 1 wherein the sulfur oxideabsorbent is effective to absorb an amount of sulfur oxides, calculatedon the basis of sulfur content, which is equal to at least about 20% ofthe absorbent's weight.
 15. The method of claim 1 wherein the sulfuroxide absorbent comprises a material of the formula:

    M.sub.2m.sup.2+ Al.sub.2-p M.sub.p.sup.3+ T.sub.r O.sub.7+r(s)

where m is about 1.1 to about 3.5; p is about 0.01 to about 0.4; M²⁺ isa divalent metal selected from the group consisting of magnesium,calcium, zinc, barium and strontium; M³⁺ is a trivalent metal selectedfrom the group consisting of cerium, lanthanum, iron, chromium, vanadiumand cobalt; T is vanadium, tungsten or molybdenum; r is about 0.01 toabout 0.2; and s is 2.5 when T is vanadium or 3 when T is tungsten ormolybdenum.
 16. An apparatus for reducing the concentration of carbonmonoxide, organic compounds and sulfur oxides in an exhaust gas from aninternal combustion engine which comprises:(a) absorber means whichcomprises a sulfur oxide absorbent for absorbing at least a portion ofthe sulfur oxides in the exhaust gas in a substantially irreversiblemanner at temperatures which are less than or equal to that of saidexhaust gas; (b) catalytic converter means for receiving effluent gasfrom the absorber means, wherein said catalytic converter meanscomprises a catalyst which is effective for catalyzing the conversion ofhydrocarbons into innocuous products; and (c) heat exchange means fortransferring heat from the exhaust gas to the catalytic converter meansby indirect heat exchange before said exhaust gas enters the absorbermeans.
 17. The apparatus of claim 16 wherein the sulfur oxide absorbentcomprises at least one metal oxide selected from the group consisting ofthe oxides of calcium, magnesium, strontium, barium and zinc.
 18. Theapparatus of claim 17 wherein said sulfur oxide absorbent additionallycomprises at least one oxide selected from the group consisting of theoxides of aluminum, silicon, copper, manganese, sodium, vanadium, iron,chromium and the rare earth metals.
 19. The apparatus of claim 16wherein the sulfur oxide absorbent comprises an inorganic oxidecomposition in association with at least one free or combined metalselected from the group consisting of vanadium, iron, lanthanum, cerium,praseodymium, samarium and dysprosium, and said inorganic oxidecomposition comprises at least one member selected from the groupconsisting of mixtures of magnesium oxide with aluminum oxide, MgAl₂O₄.xAl₂ O₃, and MgAl₂ O₄.yMgO wherein x is from 0 to 10 and y is from 0to
 10. 20. The apparatus of claim 16 wherein the heat exchange meanscomprises a first plurality of heat exchange passages which is in heatexchange relationship with a second plurality of heat exchange passagesand wherein the catalytic converter means comprises said secondplurality of heat exchange passages.
 21. The apparatus of claim 16wherein the absorber means additionally comprises a hydrocarbonadsorbent which is effective to adsorb at least a portion of the organiccompounds in the exhaust gas under the low temperature conditions whichare typical of engine cold-start and desorb them at the highertemperatures which are reached after sustained engine operation.
 22. Theapparatus of claim 21 wherein said hydrocarbon adsorbent comprises amolecular sieve component.
 23. The apparatus of claim 22 wherein saidmolecular sieve component comprises at least one material selected fromthe group consisting of faujasite, mordenite, chabazite, silicalite,Beta zeolite, zeolite X, zeolite Y and ZSM-5 zeolite.